In that incident, the FBI interviewed Mateen twice, and carried out separate interviews with other witnesses, as well as physical surveillance and checks on his records.

"Ultimately, we were unable to verify the substance of his comments, and the investigation was closed," Hopper said.

In 2014, authorities interrogated Mateen anew for "possible ties" to an American suicide bomber, Moner Mohammad Abusalha, who appeared in an al-Nusra Front video in Syria.

Hopper said the FBI determined that Mateen's contact with Abusalha "was minimal and did not constitute a substantive relationship or threat at that time".

Following those "inconclusive" findings, both investigations on Mateen were closed.

Trevor Velinor, a spokesman for the Bureau of Alcohol, Tobacco and Firearms, said Mateen legally bought the two guns believed to have been used in the attack within "the last few days".

When asked by reporters how Mateen obtained firearms despite being under the FBI radar, Hopper said: "There was nothing to keep the investigation going forward."

When pressed further, he said: "Again, investigation was closed."

Security guard

Al Jazeera's Andy Gallacher, reporting from Orlando, noted that Mateen worked as a security guard.

He said that to get the job, Mateen was required by gun laws to train with weapons, including fulfilling the required eight hours on a shooting range.

In a statement sent to the Palm Beach Post, security company G4S confirmed his employment.

"We are shocked and saddened by the tragic event that occurred at the Orlando nightclub. We can confirm that Omar Mateen had been employed with G4S since September 10, 2007," the company said.

"We are cooperating fully with all law enforcement authorities, including the FBI, as they conduct their investigation. Our thoughts and prayers are with all of the friends, families and people affected by this unspeakable tragedy."

'Nothing to do with religion'

At the press conference in Orlando, Hopper, from the FBI, confirmed that Mateen had called the police early on Sunday.

Hopper did not comment on whether Mateen declared his allegiance to Abu Bakr al-Baghdadi, the leader of the Islamic State of Iraq and the Levant (ISIL, also know as ISIS) group as reported in US media, but said the caller made references "to the Islamic State.

"We are looking into any and all connections both domestic and international," he added.

Meanwhile, in an exclusive NBC News interview Mateen's father, Seddique Mateen, said that during a visit to Miami, a neighbouring city also in Florida, the suspect became enraged after seeing two men kissing in front of his wife and their child.

The father said he was not aware of any plan by his son, adding: "This had nothing to do with religion."